Drilling fluids are categorized into water-based mud and oil-based mud. Drilling fluids are used to drill long horizontal and vertical sections of non-reservoir portions of a borehole. Water based drilling fluids designed with water, polymer needed to increase viscosity for carrying the cuttings and for fluid loss control, monovalent and multivalent salts for shale inhibition, different bridging material and weighting materials (barium sulfate, micronized barite, hausmannite, ilmenite, manganese tetroxide, hematite, and other fine, dense particles) for desired mud weight. Large amounts of weighting agents are often necessary to balance the mud weight against formation pressure. In heavily weighted fluids (greater than about 14 pounds per gallon up to about 20 pounds per gallon), large amounts of weighting agents make the dispersibility of other fluid components, particularly fluid-loss additives, difficult.
Drill-in fluids are special fluids designed exclusively for drilling through the reservoir section of a subterranean formation. The reasons for using specially designed drilling fluids include, but are not necessarily limited to (1) to drill the reservoir zone successfully, which is often a long, horizontal drain hole, (2) to minimize damage of the near-wellbore region and maximize the eventual production of exposed zones, and (3) to facilitate the necessary well completion. Well completion may include complicated procedures. Typically, drill-in fluids may resemble completion fluids. Drill-in fluids may be brines containing only selected solids of appropriate particle size ranges (for instance, salt crystals or calcium carbonate) and polymers. Usually, additives needed for filtration control and cuttings carrying are present in a drill-in fluid. As noted, drill-in fluids may contain filtration control additives to inhibit or prevent loss of the drill-in fluid into vugular zones of the permeable formation. Fluid loss involves the undesired leakage of the liquid phase of drill-in fluid containing solid particles and complete fluid losses into the formation matrix without any return. The resulting buildup of solid material or filter cake against the borehole wall may be undesirable, as may be the penetration of the filter cake into the formation. The removal of filter cake, which sometimes must be done by force, may often result in irreparable physical damage to the near-wellbore region of the reservoir. Fluid-loss additives are used to control the process and avoid potential damage of the reservoir, particularly in the near-wellbore region. Specially designed fluids may be used to be placed next to the reservoir and form a seal against fluid loss. This fluid may be different than the drill-in fluid and is often referred to as a “sealing or lost circulation pill”.
Subterranean formations having naturally occurring fractures present a problem because the fractures exacerbate undesired leakage of the liquid portion of the drill-in fluid into the formation. Thus, lost circulation fluid may be a major challenge when drilling through such naturally fractured carbonate, sandstone formations, etc.
Some lost circulation fluids are gelled, such as by gelling polymers and optionally crosslinking the resulting polymers. However, it is important to avoid premature gelation, and also important for the finally gelled material to have sufficient viscosity and strength to achieve the goal of inhibiting or preventing fluid loss. Current commercial offerings have insufficient operational robustness; examples including, but not necessarily limited to, commercial lost circulation material (LCM) technology based on polymer resin sealing or thixotropic slurry or high fluid loss pill.
It would thus be desirable to discover a drill-in fluid or a sealing pill, a drilling fluid or other fluid which would have relatively low viscosity in the drilling pipe and high dispersibility in heavily weighted (greater than about 14 pounds per gallon up to about 20 pounds per gallon) fluids but which would shortly after leaving the drill bit increase in viscosity and inhibit or prevent fluid leak-off into the formation.